In recent years, a research into a technology of smart antennas such as an adaptive antenna and a digital beam forming apparatus has been in progress.
A phase array antenna can control beam steering by using a phase shifter. However, since a price of the phase shifter is high, cost of an antenna increases. To this end, proposed is an electronically steerable parasitic radiator (ESPAR) antenna which can perform beam steering by only a manual parasitic element without requiring the phase shifter.
In general, the ESPAR antenna uses a di-pole, mono-pole, or patch-structure antenna and in the case of the di-pole-structure ESPAR antenna, a total length of the di-pole antenna is large and in the case of the mono-pole-structure ESPAR antenna, a length of the mono-pole antenna is two times smaller than the di-pole antenna, but the mono-pole antenna has a larger ground plane than the di-pole antenna. The patch-structure ESPAR antenna can be still smaller in vertical size of the antenna than the di-pole antenna and the mono-pole antenna. The patch antenna is smaller in bandwidth of the antenna and in gain than the di-pole antenna and the mono-pole antenna. Further, in the patch antenna, a horizontal size of the antenna can increase.